One of the most common air-operated pumps used in industry is a double-diaphragm, positive displacement type shown in FIG. 1. This type of pump is self-priming and displaces fluid from one of its two liquid chambers upon each stroke completion. Only several parts contact the fluid, two diaphragms which are connected by a common connecting rod, two inlet valve balls, and two discharge valve balls. The diaphragms act as a separation membrane between the compressed air supply operating the pump (air chamber) and the liquid (fluid chamber). Driving the diaphragms with compressed air instead of the connecting rod balances the load on the diaphragm, which removes mechanical stress and extends diaphragm life. The valve balls open and close on valve seats to direct liquid flow. When each diaphragm has gone through one suction and one discharge stroke, one pumping cycle has taken place. An air distribution system is part of the pump and switches the common air supply for the pump from one air chamber to the second air chamber as each fluid chamber empties at the end of its respective stroke.
The air distribution system shifts the symmetric pumping action in order to create suction and discharge strokes. When the diaphragms have traveled a maximum excursion in one direction, a mechanical pilot valve is typically actuated, shifting a main valve, and reversing the pneumatic action. The other air chamber is then pressurized to expel its fluid and the device continues this reciprocation until the air supply is stopped. Various pump manufacturers accomplish the air distribution using purely mechanical valve assemblies and/or valve assemblies that are electrically controlled.
The discharge of a double-diaphragm, reciprocating pump is dependent only on the mechanical characteristics of the air distribution system and the fluid dynamics of the pump itself. Shown in FIG. 2 is a typical discharge pressure versus time plot of a prior art, dual-diaphragm, air-operated pump. FIG. 3 shows the corresponding plot of the air distribution system connecting rod excursion in time, as the rod travels in the direction of one diaphragm pump, arbitrarily denoted as left, then to the other diaphragm pump, arbitrarily denoted as right. As the diaphragms complete their travel in one direction and reverse direction, a large pressure dip occurs when the connecting rod is at the excursion limit. This is due to the inherent pressure change when transitioning between suction and discharge strokes. The output results in a series of pulses or surges corresponding with each diaphragm pump stroke. In the control systems art, these surges manifest in the process piping are referred to as process noise. All pumps operating with some type of reciprocation produce process noise.
To reduce unwanted fluctuation, passive external pulsation dampeners can be added downstream of the pump. The prior art dampener shown in FIG. 4 contains a pressure regulator and a pressurized diaphragm acting as an accumulator. The diaphragm traps a given volume of liquid on one side and pressurized air on the other. When the fluid pressure falls in the system, the dampener supplies additional pressure to the discharge line between pump strokes by displacing fluid by the diaphragm movement. This movement provides a supplementary pumping action needed to minimize pressure variation and pulsation. Most dampeners set and maintain air pressure to match the variations in the liquid flow or discharge pressure generated by the pump. A shaft attached to the diaphragm and pressure regulator triggers the addition or deletion of the air within the air chamber side of the dampener. The dampener reacts to pressure and/or flow settings of the pump with no need for manual adjustment.
However, the prior art external pulsation dampeners are large and require additional support, making them costly to purchase and install. By their passive nature, these dampeners are slow to react and process noise is still introduced into the system as shown in FIG. 5.
What is needed is a low cost, active suppression device to anticipate and cancel process noise produced by reciprocating pumps thereby reducing water hammer and strain on equipment coupled downstream.